绑定中的NullPointerException [英] NullPointerException in binding

问题描述

您好我想根据其他对象绑定一个值。此对象为null，默认情况下将设置该值。但我仍然收到NullPointerException。我希望它会被.then（...）捕获。但事实并非如此

Hi I would like to bind a value depending other object. It this object is null the value will be set by default. But I still received NullPointerException. I expect that it will be catch by ".then(...) . but it is not the case

    relationType.bind(Bindings.when(Bindings.createBooleanBinding(() -> ( relation == null || relation.get()== null), relation))
            .then(RelationType.NEUTRAL)
            .otherwise(relation
                    .get()
                    .typeProperty()));

当我向Listener内容添加绑定时，一切正常：

All works fine when I add binding to Listener content :

    relation.addListener((observable, oldValue, newValue) -> {

    if(newValue != null) {
        relationType.bind(Bindings
                .when(relation.isNull())
                .then(RelationType.NEUTRAL)
                .otherwise(newValue.typeProperty()));
    } else {
        relationType.unbind();
        relationType.setValue(RelationType.NEUTRAL);
    }

});

但我更喜欢o nly绑定。那可能吗？

But I prefere to have only binding. That is possible ??

推荐答案

问题是 relation.get（）的时间.typeProperty（）得到评估。它在创建绑定时进行评估，而不是每次 relation 更改时进行评估。你可以使用select绑定解决这个问题，但是你会在控制台中使用这种方法收到警告：

The problem is the time when relation.get().typeProperty() gets evaluated. It's evaluated when the binding is created and not every time relation changes. You could work around this using a select binding, but you'll receive warnings in the console using this approach:

relationType.bind(Bindings.when(relation.isNull())
            .then(RelationType.NEUTRAL)
            .otherwise(Bindings.select(relation, "type"));

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